During double-jet precipitation of AgX emulsions, the excess halide ion concentration or the silver ion concentration in the reaction vessel is often controlled to match a desired profile. This is normally done by comparing the e.m.f. (mV) signals from a "silver sensor" to a desired aim value (aim mV profile) via a process controller, which then issues control commands to correct for the deviation at any given time such that the desired profile can be maintained. U.S. Pat. No. 4,933,870 by Y. Chang teaches one such state-of-the-art control system for accomplishing the above. .Typically, the silver sensor for providing the e.m.f. signals is a coated silver electrode of the second kind: Ag/AgX. The control commands are used to regulate the rate of addition of a halide salt to the reaction vessel in order to achieve the aim mV profile. This conventional practice actually controls only the excess halide ion concentration in the reaction vessel. One of the most important parameters relating to crystal growth, the supersaturation level, is not controlled. The true driving force for crystal growth is the supersaturation level which ultimately determines the size, morphology, and the composition of the emulsion crystals. It may be defined as the ratio of the product of silver and halide ion concentrations, (Ag.sup.+)(X.sup.-), to the equilibrium solubility product, Ksp: S=(Ag.sup.+)(X.sup.-)/Ksp. Since the supersaturation involves both the halide and silver ion concentrations, a conventional control system cannot control the supersaturation level in the reaction vessel.
An example of such deficiency is the renucleation phenomena in precipitations which is controlled under the identical mV profile based on the signals from a second-kind silver electrode. If the addition rate of silver and the matched salt reagents exceeds a critical value associated with the maximal supersaturation level, renucleation occurs under the same excess halide condition.
FIG. 1 illustrates, in functional block diagram form, a (prior art) control system 10 during double jet AgX precipitation. The system receives a set-point voltage Vx' at the+input of a summing node 12 from a predefined profile. The - input to the summing node 12 receives the output signal Vx from a halide sensor (not shown in FIG. 1) in a precipitation process 18 after it is amplified in an amplifier 20 and subtracts it from the set-point voltage Vx' to determine the difference there between. The difference is an error signal, Ex, the magnitude and sign of which is indicative of how far off the system output Vx is from the desired set-point and in what direction it is off. The error signal Ex is directed to a Vx controller 14, typically of the PID type, which outputs a delta correction signal .DELTA.Fx to a summing node 16. The summing node 16 also receives signal Fx which represents a predefined halide reagent flow rate profile to provide at its output the signal Fx+.DELTA.Fx, which signal is then inputted to the precipitation process 18. Another predefined silver reagent flow rate profile, FAg is also added to the precipitation process 18. The Vx signal may be further manipulated in the Vx controller 14 to achieve maximal control benefit such as is taught in U.S. Pat. No. 4,933,870. Since the halide sensor that provides the signal Vx responds only to the halide ion concentration, the signal .DELTA.Fx from the Vx controller 14 can only maintain the halide ion concentration (X.sup.-) in the mixing vessel to the desired level. The other critical component of the supersaturation, i.e., silver ion concentration (Ag.sup.+), is not monitored and controlled with this scheme.
Using the prior art (Vx) control scheme of FIG. 1, it is possible to prepare two emulsions under an identical Vx profile and yet end up with different final crystal size distributions, because renucleation will occur whenever the silver addition rate exceeds a maximal or critical value. When the supersaturation levels are monitored as disclosed in the following section, "Detailed Description of the Invention", the supersaturation of the precipitation process can be controlled by adjusting the silver addition rates to avoid the renucleation. In fact, the silver addition rate may be controlled at a profile which yields maximal growth rate of the crystals without renucleation. One of the benefits of the present invention is the reduction of precipitation time. Another benefit of the present invention is high supersaturation growth with a reduction in the width of crystal size distribution by minimizing the Ostwald ripening effect during crystal growth. It is well known that the morphology of AgX crystals such as the percent (100) face relative to the (111) face is strongly influenced by the supersaturation level in the reaction vessel. The control of the supersaturation level enables the preparation of the AgX emulsion with the desired morphology. Other critical process in the AgX emulsion preparation such as the incorporation of dopant, the recrystallization of a mixed-halide system, etc. are also known to be sensitive to the supersaturation level. The fact that the supersaturation level can be monitored in the precipitation vessel provides additional information concerning the precipitation process which is not available from conventional Vx monitoring. This additional information may be utilized to assess the reproducibility of a given process. For example, the supersaturation signals before the start of an unseeded precipitation indicates the degree of cleanness (or contamination by silver ions) of the reaction vessel. Small amounts of contamination cannot be detected by Vx measurement, but can cause variability in the result of nucleation leading to variations in the end product. The supersaturation information may be used to assess the identicality of precipitation processes when manufactured with different equipment. In scaling up a given precipitation process, this additional information can be used to assess whether the scaleability is achieved. Thus, the present invention is also an extremely useful diagnostic tool for the purpose of process monitoring and control, and additionally provides a clear advantage over conventional control practices.